The present invention relates to a magnetic producing apparatus for producing a gradient field required in, for instance, a nuclear magnetic resonance tomographic imaging apparatus and the like. More specifically, the present invention is directed to a vortex coil structure and a method for manufacturing the same.
A gradient magnetic field producing apparatus is constructed of three sorts of coils (Gx coil, Gy coil, Gz coil) for producing gradient magnetic fields along three directions (x axis, y axis, z axis) mutually perpendicular to each other. Furthermore, each of these coils along the respective directions includes an inner coil and an outer coil surrounding the inner coil capable of producing a desired gradient magnetic field as well as preventing magnetic leakage outside the gradient magnetic field producing apparatus by supplying a current to the outer coil in a direction opposite to that of the inner coil. In other words, this apparatus is a so-called active shield type gradient magnetic field producing apparatus.
FIG. 6 is a sectional view for showing such a sort of gradient magnetic field producing apparatus for the nuclear magnetic resonance tomographic apparatus, defined in a plane by the y axis and the z axis.
In this drawing, reference numeral 1 indicates a z-axial line, reference numeral 2 denotes an y-axial line, reference numeral 4 shows a Gy inner coil, and reference numeral 5 is a Gy outer coil. Reference numeral 17 shows an inner coil winding former, reference numeral 18 represents an outer coil winding former, reference numeral 19 indicates a Gx inner coil, reference numeral 20 denotes a Gx outer coil, reference numeral 21 represents a Gz inner coil, reference numeral 22 is a Gz outer coil, and reference numeral 23 indicates a fixing bolt for fixing the inner coil winding former 17 with the outer coil winding former 18. Also, reference numeral 10 shows a base member of coils (as will be discussed later).
FIG. 7 is a sectional view for showing only the Gy coil, for the sake of simplicity, defined in a plane by the x axis and the y axis. In this drawing, reference numeral 3 shows an x-axial line.
FIG. 8 is a perspective view of the Gy coils along the inclined direction. The Gy inner coil 4 is constructed of four vortex coils having the same shape, a detailed structure of which will be discussed. These four coils are arranged in a symmetrical manner with respect to the z-axial line 1, the y-axial line 2, and the x-axial line 3, and are connected in series to each other by employing connection leads (as will be explained later). Similarly, the Gy outer coil 5 is constructed of four vortex coils having the same shape. It should be understood that the vortex shapes of the Gy inner coil 4 are different from those of the Gy outer coil 5.
It should also be noted that the Gx coil is arranged in such a manner that the Gx coil is shifted by 90 degrees along the circumferential direction, and is similarly constructed by vortex coils similar to those of the Gy coil.
The Gz coil is different from the vortex coil, and can be, for example, a solenoid-shaped coil made of a plain rectangular copper wire wound on a cylindrical surface.
FIG. 9 illustrates one sheet of a vortex coil 8 used in the Gy coil. The vortex coil 8 is manufactured in such a manner that, for instance, a groove 9 having a width of approximately 1.5 mm is fabricated in a vortex form in a copper having a thickness of approximately 1 mm to 3 mm, corresponding to a conductive plate, thereby forming plural turns of coil, and further these coil turns are bent in a saddle shape on a cylindrical former. Various working (processing) methods such as machine cutting, etching, and water jetting may be introduced to form the groove 9 in the copper plate.
A description will now be made of forming the Gy inner coil 4 and the like by connecting this single body of the vortex coil 8. FIG. 10 represents the vortex coil 8 immediately after the groove has been made. As illustrated in this drawing, the vortex coil 8 adheres to a base member 10 made of an epoxy plate and the like. It should be noted that a portion of the copper plate is hatched.
This base member 10 is used to correctly bend the copper plate formed in the vortex shape, in which the groove 9 is formed. As indicated in FIG. 10, the widths of the respective turns of the vortex coil 8 are not made constant, and the copper plate is packed up to a terminal portion 16 of an inner peripheral portion. A trace or orbit of the groove 9 is determined base on the magnetic design.
FIG. 11 shows a connection construction of the Gy coil. FIG. 11A is a perspective view for indicating an overall shape of the Gy coil arranged by four sheets of the vortex coil 8, and FIG. 11B is a plan view for showing two sheets of the vortex coil 8 among them. In the drawing, reference numeral 15 is a connection lead for connecting end portions 16 of the two vortex coils 8 with each other. Since the coil connection by way of soldering does not achieve sufficient strength, brazing is generally employed. Although not shown in the drawings, the connection lead along the y-axial line 2 is connected to the outermost coil turn, and then four sheets of vortex coil 8 are mutually connected series to each other, so that the Gy inner coil 4 and the Gy outer coil 5 are formed.
Since the conventional gradient magnetic field producing apparatus has been arranged as described above, there are various problems mainly caused by the specific conditions of the manufacturing stages. One problem is that when the connection lead 15 is jointed to the vortex coil 8 by way of brazing, since the copper plate is packed up to the terminal (end) portion 16 of the inner peripheral portion of the vortex coil 8, it is difficult to pick up the terminal portion 16 This, there may be insufficient work space to secure a brazing head portion on the connection portion.
Also when the vortex coil 8 of the copper plate is bent, since the inner peripheral terminal portion 16 is highly rigid the terminal portion 16 cannot be smoothly fitted on the curved surface. Therefore, there is another problem that the inner peripheral terminal portion 16 may rise from the curved surface.
Moreover, when the vortex coil 8 of the copper plate is bent, the base member 10 must be employed to mechanically reinforce the copper plate by adhering to the copper plate so as not to positionally shift these coil turns. Then, when the gradient magnetic field producing apparatus is of a so-called "active shield type", due to the presence of the reinforcing base, a difference between the diameter of the inner coil and the diameter of the outer coil would is less than it would be in the absence of the reinforcing base.
As previously explained, since the inner coil and the outer coil are employed so as to produce the magnetic fields directed oppositely inside and outside the apparatus, there is a difficulty that a higher magnetomotive force is required, as compared to the inner coil only, in order to produce a desired magnetic field as the gradient magnetic field producing apparatus. That is, assuming now that both of the maximum inner diameter of the inner/outer coil assembly and the minimum outer diameter thereof are limited to constant values thereof, each other, the dimension between the inner coil and the outer coil is reduced, so that the required magnetomotive force would be increased.
In other words, this sort of active shield type gradient magnetic field producing apparatus must be designed in accordance with the following basis. That is, the inner coil must be mounted, while being separated from the outer coil as far as possible within the allowable space of this field producing apparatus, so that the field producing efficiency is increased and the overall required magnetomotive force is reduced. When the magnetomotive force is lowered, since the rated current and the rated voltage of this apparatus are lowered, the power source capacity may be reduced, so that the vibrations, noise and heat generation of this gradient magnetic field producing apparatus could be reduced. As a consequence, the employment of such a base member 10 would impede the small-sizing in case of this type of gradient magnetic field producing apparatus.